DxOMark measurements for lenses and camera sensors

What is noise?

Noise in an image is described as random granulation that
is particularly visible in uniform areas.

Noise in a landscape photo. Note that the noise is considerably more noticeable in the more uniformly-colored patch of sky in the enlargement at right.

Origins of noise

Noise in an image can stem from several different factors:

Photonic
noise: Classical light sources produce random variations in luminous flux
which, after deflection in the optical system, produce the most common
form of noise in photographic images.

Thermal
noise: Sensors can generate random signals due to the effect of ambient
temperature.

Transfer
process noise: The process of charge transfer may be incomplete or
interfere with adjacent photosites, thereby generating noise.

Definitions and units

Noise itself is characterized by several values, among which
are Standard Deviation and Signal-to-Noise Ratio (SNR). Other measures are derived
from these, such as
Dynamic Range and Tonal Range. (Other aspects of interest
include color sensitivity, and noise granulation and coloration, which will be discussed separately on
this site.)

1. Standard Deviation

Standard
deviation of noise represents the dispersion of gray levels around the
mean gray level, and is expressed in gray levels (GL).

,

where is
the mean gray level measured in the patch.

2. Signal-to-Noise
Ratio (SNR)

SNR defines the purity of the output signal and gives direct
information about the camera’s ability to reproduce a signal. On a
logarithmic scale, it is expressed in decibels (dB), and is obtained from
the following formula: .

SNR is independent of any gain applied to the signal since signal and noise are equally amplified.

3. Dynamic Range

Dynamic range is defined as the ratio between the highest and
lowest gray luminance a sensor can capture. However, the lowest gray luminance
makes sense only if it is not drowned by noise, thus this lower boundary is
defined as the gray luminance for which the SNR is larger than 1. The dynamic
range is a ratio of gray luminance; it has no defined unit per se, but it can
be expressed in Ev, or f-stops.

4. Tonal Range

Tonal range is the effective number of gray levels the system
can produce. This measure has to take noise into account (indeed, a very thin
gray-level quantization is irrelevant if the quantization step is much smaller
than noise). The standard deviation of noise can be viewed as the smallest
difference between two distinguishable gray levels. The expression of the tonal
range is

.

Since tonal range is a number with no unit, one can consider
instead ,
which represents the number of bits necessary to encode all distinguishable
gray levels.

Further readings for the DxOMark measurements for lenses and camera sensors

To provide photographers with a broader perspective about mobiles, lenses and cameras, here are links to articles, reviews, and analyses of photographic equipment produced by DxOMark, renown websites, magazines or blogs.

On DxOMark, we evaluate and rank many types of digital cameras with image sensors that vary widely in pixel count, pixel size, and digital signal processing. To ensure that sensor performance comparisons between cameras are fair, it is very important both to test under identical shooting conditions and to take viewing conditions into account.

The Nikon D5000 and the Canon EOS 500D were two main releases for spring 2009. Launched at the same time, and targeting the same market, these two cameras were the perfect subjects to use for an extensive demonstration of the DxOMark Database.